This invention pertains to improvements in sewing thread, and more particularly to an improved continuous multifilament synthetic sewing thread which is characterized by the ability to withstand the relatively high temperatures generated by high speed industrial sewing machines.
Sewing threads formed of continuous multifilament synthetic yarns are known and have been used heretofore in certain sewing applications. This type of thread construction is generally less expensive to produce than sewing threads of conventional spun or corespun construction. However, the continuous multifilament sewing threads heretofore available have had certain inherent limitations which have limited their usefulness to a limited range of specific, relatively non-demanding applications. For example, when such threads are sewn into a seam, they generally present a relatively shiny appearance quite different from that presented by sewing threads of a spun yarn or corespun construction. Such an appearance is unacceptable in many types of garments where the appearance of the stitch is highly important. Additionally, in seams formed with continuous multifilament sewing threads, individual filaments sometimes tend to separate from the remainder of the filament bundle, presenting an unacceptable fuzzy appearance. Another very significant limitation of the continuous multifilament sewing threads heretofore available has been that such threads have been incapable of withstanding any significant level of heat generated during the sewing operation. Consequently, such threads have been used primarily on lightweight fabrics and in applications where little heat is generated.
Perhaps one of the most demanding applications for a sewing thread is in the sewing of relatively heavy weight fabrics, e.g. bottom weight fabrics such as denims or corduroys. In forming seams in garments of such fabrics, it may be necessary to sew through as many as four to six plies of fabric. At the high speeds used in industrial sewing operations, very high temperatures are produced at the sewing needle when sewing such fabrics. The needle temperature may rise for example to 500 to 600 degrees F. or higher, sufficient to melt a synthetic sewing thread. In attempting to deal with the problems presented by such high needle temperatures, various efforts have been undertaken, such as directing compressed air at the needle for cooling, as well as various special needle designs specifically intended for cooling. Because of the extreme heat at the needle, bottom weight fabrics are typically sewn with a sewing thread of cotton sheathed corespun construction on the needle. On the looper, where the temperature is not as severe, threads of conventional cotton spun yarn construction are typically used. Synthetic sewing threads of a continuous multifilament construction have heretofore been unsuitable for use in these applications because of the inability to withstand the needle heat which is generated.